Conventionally, in a smoke sensor of the reflection type which is used for performing fire monitoring in a wide area, a reflector plate is opposed to a smoke sensor main unit having a light projection device and a light reception device, being separated from the main unit by a predetermined monitored distance, for example, several tens of meters. A fire is detected on the basis of attenuation of light received from the light projection device which is caused by smoke entering the monitored space.
In this case, for example, a near infrared LED is used as a light emitting element for the light projection device. Light emitted from the near infrared LED is converged into beam light by a condenser lens. The beam light impinges on the reflector plate which is opposed to the light projection device being separated therefrom by the predetermined monitored distance, and is reflected thereby. The reflected light impinges on the light reception device, and a fire is detected on the basis of light attenuation due to smoke entering the monitored space.
In such a smoke sensor of the reflection type, the light projection device converts light from the near infrared LED into parallel beam light by using the condenser lens, and then emits the light into the monitored space. The beam light from the light projection device makes a round trip from the main unit and the reflector plate, and then impinges on the light reception device. In the case where the monitored distance between the light projection device and the reflector plate is as long as, for example, 40 meters, when the beam light reaches the reflector plate, the beam image is largely expanded by light diffusion. When the light reflected by the reflector plate returns to the light reception device, similarly, the beam image is largely diffused. Therefore, the light reception device can detect only a very small portion of the energy of the emitted light beam.
It has been reported that, even after a device is installed, a side wall of a building undergoes little temporal distortion. If the light intensity distribution in a section of the beam is not uniform, when a portion of a low light intensity is caused to impinge on the reflector plate by the distortion of the side wall, the light reception signal which is generated in the case where no smoke exists is very weak in level, with the result that a sufficient S/N ratio cannot be obtained. Furthermore, there arises a problem in that the maximum monitorable distance is shortened. Therefore, it is preferable that the light intensity distribution in a section perpendicular to the optical axis of beam light is made as uniform as possible.
In order to solve the problem of the nonuniform intensity distribution of beam light, for example, a light projector shown in FIG. 9 has been proposed (Japanese patent publication (Kokai) No. HEI5-79979). Referring to FIG. 9, light from a light emitting diode 105 in the light projector is introduced into a waveguide 103 by an imaging lens 104, so as to propagate in the waveguide 103, whereby the energy distribution is uniformalized. Light emitted from the end face of the waveguide 103 is imaged at a distant position by a projection lens 102.
In the structure of such a light projector which uniformalizes the energy distribution of a projection beam, the imaging lens, the waveguide, and the projection lens must be arranged in front of the light emitting diode. Therefore, the optical system for uniformalization is relatively complex, and the dimension in the optical axis direction is increased. As a result, the structure has a disadvantage that the light projector is bulky.
The invention has been conducted in view of the problems of the prior art. It is an object of the invention to provide a light projection device for a photoelectric smoke sensor in which beam light from a light emitting diode can be uniformalized in a beam section direction by a simple optical structure so as to compensate an optical axis deviation.